A conventional cutting tool holder holds a cutting insert. During a machining operation, the cutting insert becomes heated. The heat spreads quickly through the cutting insert. The cutting insert, which is generally formed of cemented carbide, reaches, in a very short time, a range of temperatures within which the resistance to plastic deformation of the cutting insert material decreases. When large cutting forces act on the cutting insert, this phenomenon entails a risk that the cutting insert will be subject to plastic deformation, in particular, in the proximity of the cutting edge, where insert breakage can result. In order to diminish the risk of plastic deformation, an efficient system for cooling the cutting insert is required, whereby the working temperature of the insert can be regulated within desired limits. Generally, the cutting insert and the surrounding so-called cutting zone, are cooled by a coolant, in fluid form, that is led from outside the tool holder to flow towards the area where the heat is generated. An example of such fluid flow is described in U.S. Pat. Nos. 6,045,300 and 6,299,388. Such a supply of coolant has been generally arranged from above and directed downwardly towards the cutting insert and the chips, which are broken against a chip breaking upper side of the cutting insert. This method of supply, however, results in only a very limited amount of the coolant having any practical affect on the cutting insert. That is, because of the presence of the chips, the cutting edge is only exposed to the coolant to a very limited degree.
Another means of supplying coolant in fluid form is to steer the medium in a direction between the chip breaking surface of the cutting insert and the chip itself, as described, for example, in U.S. Pat. No. 6,652,200. However, when the cooling liquid is applied at normal pressures, this procedure does not result in any significant improvement in cooling effect compared to the aforementioned methods because the cooling liquid does not reach that part of the cutting insert which is hottest and which is exposed to the greatest mechanical load. Thus, a clear risk exists of the cutting insert becoming plastic because of excessively high temperatures. The effect of the cooling can certainly be increased considerably by increasing the pressure of the cooling liquid to very high levels but the equipment required to raise the cooling liquid pressure to this level is very complicated and expensive. Working with extremely high fluid pressure is furthermore, in practice, is undesirable.
In order to overcome the problem indicated above it has been proposed that the cutting insert itself be cooled from within, with the prime aim of holding the temperature in the cutting insert at such a low level that the risk of plastic deformation is for all intents and purposes eliminated. Several different solutions to the problem of how to internally cool cutting inserts have been proposed. For example, U.S. Pat. No. 5,275,633 describes a cutting insert which comprises two identical, partial bodies which are sintered together in such a way as to form internal, open channels through which the coolant can flow. German Patent No. 3,004,166 describes a cutting insert with a transverse hole through which a coolant can pass from an underlying shim in the direction towards a separate cover plate on the upper side of the cutting insert in order to be finally directed towards the cutting edge of the insert. French Patent No. 2,244,590 describes a cutting insert with a transverse channel for the coolant which extends from the underside of the cutting insert to the upper side where it discharges in the immediate vicinity of the cutting insert's cutting edge. U.S. Pat. No. 5,439,327 describes a cutting insert having an open groove disposed in the clearance surface of the cutting insert for the transport of a coolant from a channel in the corresponding tool holder in the direction of the cutting insert edge. U.S. Pat. No. 5,237,894 describes a cutting insert with a transverse, open channel for cooling liquid which terminates in an opening on the upper side of the cutting insert.
Common to all the solutions to the problem indicated above, which are based on transverse or open channels or grooves for feeding the coolant, is that the channels weaken the cutting insert in the cutting edge area and there is a risk that the channels will be blocked by the hot material of the chip which sticks to the insert surface. Furthermore, the existence of the channels or grooves limits the possibilities to design the cutting insert with an optimal chip breaker geometry.
It should also be mentioned that U.S. Pat. No. 3,571,877 describes a cutting insert with an internal cavity in which a coolant can circulate. However, this cavity weakens the cutting insert to such a large degree that its use is not practical when exposed to typical cutting forces.
It is desirable in the machine tool art to be able to provide an improved cutting tool with an internal cavity in which a coolant can circulate without weakening the cutting tool. This would result in a longer life for the insert.